With the aid of histocompatibility markers, cellular composition and interactions can be determined in normal and neoplastic tissues derived from genotypically mosaic mammary glands using in vivo transplantation techniques. Three separate populations of cells representing 3 different cell types, having separate origins in adult tissues, comprise normal glands and give rise to 3 different forms of neoplasia. The etiology, immunology, hormone responsiveness, other biological characteristics, and histology are different for each type of neoplasia. I plan to establish a reliable system of classification of breast neoplasia based on cellular composition and cell type of origin. This could lead eventually to accurate analysis of biological characteristics of human breast cancer. Allogeneic normal mammary and syngeneic early neoplasms with specific antigens grow and survive indefinitely in immunologically normal hosts when the stroma of the implant is syngeneic. Studies using these tissues will provide information on host immune responses during the genesis of neoplasia and could lead to improved methods of immunoprophylaxis.